Is there a way to add an additional charge or fee for extra services to a current stay after they've booked?

I have a client who requested I make an appointment and take their two dogs to their grooming salon on their last day right before pick up. I believe I should be compensated for at least gas, if not also for my time. I was wondering if there is any way I can add the extra fee to the booking or would I have to ask for cash? Also, same question applies if a client wants to pay for a bath after stay has been booked. Thanks!